1. Field of the Invention
The present invention relates to a method for producing a resin sealed type semiconductor device.
2. Description of the Prior Art
FIG. 1 shows the structure of a resin sealed type semiconductor device, to which the present invention is to be applied, and FIG. 2 shows a circuit assembly composed of a semiconductor chip and a lead frame. Referring to the Figures, a lead frame 1 used for assembling the semiconductor device comprises a die pad 1a, which also functions as a heat sink, a lead 1b, and a semiconductor chip 2 which is mounted on the main surface side of the die pad 1a. The semiconductor chip 2 is connected with the lead 1b of the lead frame 1 through a bonding wire 3, thereby forming the circuit assembly shown in FIG. 2. The circuit assembly is molded, and resin sealed with a resin package 4. Such a semiconductor device is normally mounted on a heat sink 5 with the thermal transfer resistance between the die pad 1a and the heat sink 5 reduced to as low of a value as possible. The resin package 4 has a resin layer thickness d coating the rear surface side of the die pad 1a. Resin layer thickness d is extremely small compared to a resin layer on the main surface side and is, for example, formed to a thickness of approximately 0.5 mm.
Outer shapes of the package of a resin sealed type semiconductor device are standardized, for example, by the U.S. JEDEC (Joint Electron Device Engineering Council), and package outer shapes such as T0220, T03P, and the like are known. The present invention relates to an improvement of the production method for a resin sealed type semiconductor device, and particularly, is directed to a production method for resin sealed type semiconductor devices with package outer shapes having resin packages with portions of different resin layer thicknesses such as T0220, T03P, and the like.
Molding of the resin package 4 is generally achieved by transfer molding. In the transfer molding process, a lead frame assembly as is shown in FIG. 2 is preheated to a predetermined temperature. The lead is pinched between an upper die and a lower die of a molding die heated to a predetermined temperature. The die pad is inserted into a cavity formed between the upper and lower dies, and a sealing resin is injected through a gate of the molding die to achieve molding. Heretofore, when an epoxy type resin is used as a sealing resin, the surface temperature of the die is kept at 170.degree. to 180.degree. C. for molding.
When the resin package 4 as shown in FIG. 1 is molded using a transfer molding die, since a cross section between the die pad 1a and the cavity wall of the die at a rear surface side of the die pad 1a is extremely thin compared to the main surface side, resin flow at the rear surface side in the cavity is disturbed compared to the main surface side under the same molding pressure. Thus the sealing resin injected through the die gate tends to flow and fill the main surface side of the cavity earlier than the rear surface side. As a result, the filling of resin at the rear surface side of the die pad is insufficient, resulting in molding defects such as pinholes, weld marks, and the like.
FIG. 3 shows such a condition. Sealing resin is shown injected through a gate 8 at the left end of a molding die comprising an upper die 6 disposed at the main surface side of the die pad 1a and a lower die 7 disposed at the rear surface side. The sealing resin fills the main surface side in the cavity earlier where the flow path cross section is larger, and resin filling is delayed at the rear surface side in the cavity where the flow path cross section is small. Therefore, fusion of the resin does not occur between the sealing resin R flowing through in the rear surface side and the sealing resin R flowing through the main surface side to the rear surface side from an end at the right side, and defects such as weld marks, pinholes, and the like tend to be generated. Further, if the above-described defects such as weld marks, pinholes, and the like occur at the rear surface side of the die pad, strength of the resin package decreases, leading to breakage and cracking.
There are known methods such as disclosed in Japanese Patent Application Laying-open No. 290032/1990 and Japanese Patent Application Laying-open No. 94432/1991, in which a control member, such as an orifice, is provided at the die side or the lead frame side for slowing the flow of resin to the main surface side of the die pad in the cavity of a die so that sufficient sealing resin flows to the rear surface side of the die pad.
However, these methods require complex dies, resulting in an increase in cost and limitation in the applicable package outer shape.